1. Field of the Invention
The present invention relates to a tone correcting apparatus for correcting tone of a picture displayed by a laser display apparatus which modulates a laser beam by using light modulators such as an optoelectronic modulator, an acoustooptic modulator or the like.
2. Description of the Prior Art
Various types of color laser display apparatus have been proposed so far. Fundamentally, a color laser display apparatus is constructed as shown in FIG. 1. FIG. 1 shows a projection type color laser apparatus that is described in "Image Display" pp. 183 to 184, written by Iwao Ohishi, Takatoshi Ohkoshi, Norihiko Nakayam and published by Corona Publishing Limited.
As shown in FIG. 1, there are provided laser sources 1A and 1B. A laser beam from the laser source 1B is split by a spectral prism 2 into blue (B) and green (G) laser beams. The green laser beam (G) is introduced through a light intensity variable attenuator ATT into a light modulator 3G for the green laser beam. The blue laser beam, split by the spectral prism 2, becomes incident on a light modulator 3B for the blue laser beam (B). Similarly, the red laser beams (R) from the laser source 1A is introduced into a light modulator 3R for the red laser beam (R).
As the light modulator 3 formed of these light modulators 3R, 3B, 3G for the red, blue and green laser beams (R, B, G), there are utilized such one that an electric field dependency of birefringence such as KDP (potassium dihydrogenphosphate), DKDP (double potassium dihydrogenphosphate), ADP (ammonium dihydrogenphosphate) or the like is utilized as the optoelectronic modulator to modulate a light by a video signal or the like or that the acoustooptic modulator is used to modulate a light. The acoustooptic modulator makes effective use of the fact that a cyclic change of a refractive index caused by ultrasonic waves propagating a transparent medium acts as a kind of diffraction grating relative to an incident laser beam so that the diffraction of laser beam occurs. Intensity of diffracted light, intensity of sound wave and the emitted angle of diffracted angle are dependent on the frequency of ultrasonic wave. As the acoustooptic modulator, there is constructed a diffraction grating by the cyclic change of refractive index having a cycle of wavelength sound wave that takes place when ultrasonic waves propagate into crystal materials such as TeO.sub.2, PbM.sub.0 O.sub.4 or the like.
R, B and G video signals from a light modulator drive amplifier 4 are supplied to these light modulators 3, in which the laser beams are optically modulated by these video signals R, B and G.
Video signals R, B, G from a camera or the like are processed in a matrix fashion by a chromaticity conversion matrix circuit 7. Then, the video signals are compensated in aperture by an aperture compensator 6 and then supplied through a nonlinear amplifier 5 to the light modulator drive amplifier 4. The video signals R, B, G from the light modulator drive amplifier 4 are added with bias voltages from a black level fixing circuit 8.
Modulated laser beams from the light modulators 3R, 3B and 3G are reflected by beam splitters (BS) 10RB, 10BB, 10GB after passing through polarizers 9R, 9B, 9G for the red, blue and green laser beams and introduced into the black level fixing circuit 8 which then determines the bias voltages of the video signals R, B, G. Laser beams passed through the beam splitters 10RB, 10BB, 10GB are traveled through dichroic mirrors (DM.sub.1, DM.sub.2) 10R, 10GB and a mirror (M.sub.1) 10G and then chopped by a light chopper 11. These laser beams are traveled through a mirror (M.sub.2) 12 and then introduced into a two-dimensional scanner 13 that constructs horizontal and vertical deflectors.
The laser beam reflected by the mirror (M.sub.2) from the light chopper 11 is introduced through a cylindrical lens 14 (SL.sub.1), which is used to correct an irregular scanning line pitch, into a polygon mirror (RM) 15 serving as a horizontal deflector. Laser beams reflected by the polygon mirror 15 are traveled through a first relay lens (L.sub.1) 16, a cylindrical lens 17 and a second relay lens (L.sub.2) 18 and then focused on a galvano meter (G) 19 serving as a vertical deflector. These laser beams are deflected in vertical direction by the galvano meter (G) 19 and then projected through projection lenses (L.sub.3, L.sub.4) 20, 21 on a screen 22 as a picture.
The aforesaid color laser display apparatus employs the light modulators 3 (3R, 3B, 3G) and these light modulators 3 employ the optoelectronic modulators or acoustooptic modulators. However, there is then the problem that a relationship between modulated laser beam and a modulated voltage presents a distortion as shown by a sin.sup.2 curve 23 in FIG. 2. As a method that is known most to correct such distortion, it is known that a distortion is corrected in an analog fashion by using a polygonal line approximating circuit formed of diodes or the like to thereby correct a tone of a picture displayed on a screen.
FIG. 3 shows in principle an arrangement of a polygonal line approximating circuit formed of diodes. As shown in FIG. 3, a resistor R.sub.0 is connected to a current source I.sub.1 in parallel. A series circuit of a diode D.sub.1, a reference voltage source E.sub.1 and a resistor R.sub.1 is connected to the current source I.sub.1. Similarly, a series circuit of a diode D.sub.2, a reference voltage source E.sub.2 and a resistor R.sub.2 and a series circuit of a diode D.sub.3, a reference voltage source E.sub.3 and a resistor R.sub.3 are respectively connected to the current source I.sub.1 in parallel. Then, a relation between the current source I.sub.1 and an output voltage V.sub.2 can be presented as a polygonal line curve 24 shown in FIG. 4 because a voltage versus current characteristic of diode becomes an exponential function. If angles .theta..sub.0 to .theta..sub.3 of polygonal line is selected so as to provide a predetermined tone, then the predetermined polygonal line curve can be obtained.
It has been known that the polygonal line approximating circuit formed of a plurality of resistors and diodes and a measuring apparatus for monitoring a tone characteristic of the laser display apparatus are provided on the outside of the laser display apparatus to correct a tone. Japanese Laid-Open Patent publication No. 3-116088 describes such a proposal that, when a laser display apparatus is assembled, a measurement is carried out by using a luminance signal for adjustment and a tone is corrected by using a ROM (read only memory) in which there is stored a luminance correction value calculated from a measured value.
If the conventional polygonal line approximating circuit using the diodes or the like fine corrects tone, then diodes D.sub.1, D.sub.2, D.sub.3, . . . and resistors R.sub.1, R.sub.2, R.sub.3, . . . corresponding to the number of polygonal lines are required, which unavoidably increases the number of assembly parts in the polygonal line approximating circuit. Also, upon adjustment, voltages applied to the diodes corresponding to the polygonal lines approximated must be adjusted. In addition, the adjustment of such respective voltages cannot be independently made, which then make the adjustment complicated.
In order to correct tone o#each laser display apparatus strictly, there is required an external measuring apparatus which monitors tone characteristics. Further, since a tone characteristic of the same laser display apparatus is changed due to an aging change of a light modulator, correction data obtained in the assembly process is not sufficient. Hence, tone characteristics are measured periodically by the external measuring apparatus and must be corrected. Furthermore, according to the conventional tone correcting apparatus, the tone cannot be corrected automatically while a picture of a video signal is being projected onto a picture screen under normal condition.